TULSA, Okla. — A 16-year-old Oklahoma boy will stand trial as an adult in the stabbing deaths of his parents and three siblings, a judge ruled Monday, rejecting a request by defense attorneys to certify him as a juvenile delinquent or youthful offender.

Michael Bever and his 18-year-old brother Robert Bever are charged with first-degree murder in the July 22 deaths and have pleaded not guilty.

Attorney Rob Nigh said he would appeal Special Judge Martha Rupp Carter’s decision to the Oklahoma Court of Criminal Appeals and, if necessary, the U.S. Supreme Court.

“He’s a child,” Nigh said after Monday’s hearing. “The reality of the situation is we should not have children going to the state penitentiary.” Nigh said even though Bever is charged with first-degree murder, “that doesn’t change his biology.”

At a hearing Friday, Nigh suggested Michael Bever was struggling with mental health issues.

Tulsa County District Attorney Steve Kunzweiler said Bever acted as an adult by conspiring with his brother to kill their father, David Bever, and mother, April, along with siblings Daniel, Christopher and Victoria.

“Oklahoma law is very clear: if anybody’s going to commit a crime of murder in the first degree, you’re going to be tried as an adult,” Kunzweiler said after the hearing.

Autopsy reports released Monday by the Oklahoma Medical Examiner’s Office show the five family members were stabbed dozens of times and died of multiple, sharp-force injuries. The report noted that David Bever had at least 28 total wounds to his body; April Bever had at least 48 total wounds.

If convicted, Michael Bever faces life in prison without the possibility of parole.

Authorities have not spoken about a motive in the case. According to a search warrants affidavit, one of the brothers told police that plans for the killing were on a thumb drive inside the family’s Broken Arrow home.

Two family members survived the attack and one of them, a teenage girl, plans to testify for the prosecution at the brothers’ Jan. 22 preliminary hearing. Neither brother was present in court Monday.